HVAC-Talk site will be slow for the next few days. It's normal site/server maintenance. Thx -Dad

Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

I am trying to understand ductwork sizing as it relates to residential AC. I have read Manual D cover to cover but sections 3 and 4 make no sense to me, I just don’t get it, call me stupid but those sections are very hard to understand the way it’s is written.
Are there any suggestions for other written material that explains those two areas? Thanks

I have one..Question.Lets say you go to a house that has three story.one unit and only one distribution system.

You head to the third floor to see if you are getting any air.You have lots of noise and what appear to be good air coming out of the vents.

So knowing that the duct are to small,because you have the jet noise in the house,you decide to change the duct to a
bigger size to reduce the noise.
But you can only change the ducts in the first floor,which
you do so.

Now you have sucessfully reduce noise,but you have created another problem on the third floor because not only is there
no noise,theres no air as well.

What if I only increase the size of the duct main trunk,supply and return just before it ties into the riser
on first floor.Total of 30 feet for both supply and return
coming off the package unit.
No change on the sizes of the outlets and only thing added was a 12inch return.Which added another 25 feet.

Originally posted by jrc2905 What is confusing how to figure the fan operating point?

Manual D,shows fan curves,most all residential "fan Data" from mfrs. is a chart,not a curve as shown in Man.D.

Look at your mfrs fan data ,if it's a chart ,convert the chart to a curve for one speed,then the other speeds.This should help you understand.Just use a sample chart and plot the points on the fan chart,then connect the dots into a "curve".

When you get to the actual Friction Loss worsheets,to determine the FR,you see that all you need is the cfm at what ESP,from the fan chart.

My guess is that mfrs Fan data ,was in the form of fan curves ,years ago.Today the only fan curves a see is for Variable speed fans.

Men,
as a HO, I'm very impressed with the knowledge you have about your craft. You definitely deserve the rates you charge! Dash can you install my system for me? Just kidding, I'm sure the person I have is just as qualified.